U.S. Pat. No. 5,427,797, issued Jun. 13, 1995 in the name of Adrian et al., describes a rotary-driven acoustic catheter for ablation and angioplasty use. As described therein, a swash-plate-like rotary element located near the distal end of the catheter is driven by a shaft, which, during operation, is in turn driven by a rotary motor. The swash plate element rotates, and a bearing surface portion, which may be its distal surface, describes a reciprocal axial motion. A follower element, which is coupled at its distal end to the biological medium which is to be treated, is biased toward a proximal position by a spring arrangement. Rotation of the rotary driver causes the driver bearing surface to repeatedly urge the follower in a distal direction against the spring force.
The arrangement as described in the abovementioned Adrian et al. patent is capable of generating acoustic energy in the biological medium at a frequency which is a multiple of the rotational rate of the shaft and drive element, in those cases in which the bearing surfaces have more than one round-trip axial excursion for each unit of revolution. In one illustrated embodiment, the rotational driver has a "wavy" surface which provides two back-and-forth axial excursions for each rotation of the rotary driver. The apparatus of the abovementioned Adrian et al. patent, when driven at 200,000 rpm, is capable of generating an acoustic frequency in excess of 6 KHz. without doubling of the frequency by a multiplying driver, and in excess of 13 KHz. when using a doubling-type rotating driver arrangement. Multiplying factors greater than two are described.
When arrangements such as those described in the abovementioned Adrian et al. patent are operated, frictional forces tend to create a substantial amount of heat. Fluid flow through the catheter tends to carry some of the heat away, but it is not always desirable to allow fluid flow. When operated for long periods of time at rotational rates or at high frequencies of operation, heat energy can build up in the region of the rotary-to-axial motion converter, and result in high temperatures. Such high temperatures may adversely affect the operation, or may tend to cause premature failure of the converter. In addition, an excessively high temperature of the converter may injure the patient. Improved acoustic catheters are desired.